FR2712724A1 - Heterogeneous fast neutron reactor core design - Google Patents

Abstract

In a heterogeneous fast reactor core, in which a reduced number of target fuel assemblies (12) contg. lower actinide nuclides are dispersed and charged in heterogeneous manner, the novelty is that: (a) the casing tube (14) of each target fuel assembly has the same shape and size as the hexagonal casing tube of an ordinary fuel assembly; (b) each target rod (18) contains 20-50% lower actinide nuclides and has a smaller diameter than an ordinary active rod; (c) the number of charged ordinary active rods in an ordinary fuel assembly is 271, while the number of charged active target rods in a target fuel assembly is 331 or 397; and (d) 30-50 of the target fuel assemblies are dispersed and charged in heterogeneous manner in the reactor core.

Description

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  The present invention relates to a fast neutron reactor core in which 30 target assemblies, each containing 20 to 50% of nuclides into minor actinides having long radioactive half-lives, are dispersed and heterogeneously charged with a in such a way that these nuclides can be burned efficiently with no effect

  considerable on the characteristics of the heart.

  The use of a fast neutron reactor to annihilate nuclides into minor actinides with a long radioactive half-life, which are present in the nuclear fuel consumed in

  Light water reactors are now the subject of studies.

  These minor actinides include, for example, the

  neptunium (Np), americium (Am), and curium (Cm).

  To charge a fuel containing nuclides with minor actinides in the core of a reactor, there are two approaches: one called homogeneous loading of the fuel in which small quantities of nuclides in minor actinides are added and charged with homogeneously throughout the core fuel, and the other is referred to as heterogeneous fuel loading in which a small number of special fuel assemblies (called target-fuel assemblies) with the addition of large quantities of nuclides in minor actinides, are charged

in the reactor core.

  It has now been found that the homogeneous loading of the fuel makes it possible to annihilate the nuclides in minor actinides without having a serious effect on the characteristics of the heart, if the content of nuclides in minor actinides is reduced to approximately% of the average. of the reactor. Since the nuclides in minor actinides, americium and curium, produce large quantities of neutrons, gamma rays and heat, it is however necessary to use cells with thick shielding for the fabrication of fuel.

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  On the contrary, the heterogeneous loading of the fuel makes it possible to reduce the number of fuel elements containing nuclides into minor actinides, and therefore has certain advantages in terms of fabrication and management of the fuel. By using heterogeneous fuel loading to load the target fuel assemblies of the reactor core with a high content of minor actinides of nuclides (i.e. with nuclides of minor actinides contained in quantities from 20 to % of the average of the assemblies), it can be seen that the nuclear and thermal properties are improved by the dispersed loading of the fuel rather than by the

concentrated loading of it.

  However, the physical properties of nuclides to minor actinides are expected to render the melting point and thermal conductivity of fuel containing large quantities of nuclides into lower actinides, lower than that of ordinary mixed oxide fuel (MOX). When target-fuel assemblies containing nuclides of minor actinides are simply loaded by the heterogeneous loading technique, it is expected that the allowable linear power of the fuel will be exceeded. To avoid this, it is necessary to lower the operating power and, therefore, it is practically impossible to obtain sufficient annihilation of the nuclides into minor actinides in

the fuel-target assembly.

  An object of the present invention is therefore to provide a solution to the technical problems set out above by providing a neutron reactor core.

  fast in which a reduced number of assemblies-

  targets containing large amounts of nuclides in

  minor actinides with long radio half-lives

  assets can be dispersed and loaded heterogeneously to effectively burn large

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  amounts of nuclides to minor actinides without effect

  important on the characteristics of the heart.

  According to one aspect of the present invention, there is provided a heterogeneously charged type of fast neutron reactor core, in which a reduced number of target assemblies containing nuclides of minor actinides are dispersed and heterogeneously charged, in which: - a tube-shell for each of said fuel-target assemblies has the same shape and the same dimensions as a hexagonal tube-shell for an ordinary fuel assembly of a core, - each of the active target bars located at the the interior of the tube-envelope contains 20 to 50% of nuclides in minor actinides and has a diameter smaller than that of an ordinary active rod, and - the number of ordinary active bars loaded into the ordinary fuel assembly of the core is 271 then that the number of active target bars loaded in the fuel-target assembly is 331 or 397,

  - 30 to 50 of these fuel assemblies -

  targets being dispersed and loaded heterogeneously

  in the core of the fast neutron reactor.

  Preferably, the fuel base material containing nuclides of minor actinides is a mixed oxide or uranium and plutonium nitride. According to another aspect of the present invention, there is provided a heterogeneously charged type of fast neutron reactor core in

  which a reduced number of fuel assemblies-

  targets containing aluminum oxide, magnesium oxide, aluminum magnesium oxide, cerium oxide, gadolinium oxide, aluminum nitride or nitride of zirconium, to which nuclides are added to minor actinides, are dispersed and loaded heterogeneously, where:

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  - an envelope tube for each of the target fuel assemblies has the same shape and the same dimensions as a hexagonal envelope tube for an ordinary fuel assembly of a reactor, - each of the active target bars contained in the envelope tube contains 20 at 50% of nuclides into minor actinides, and has a diameter smaller than that of an ordinary active core bar, and - the number of ordinary active core bars loaded into the ordinary fuel assembly is 271, while the number of active target bars loaded in the target fuel assembly is 331 or 397,

  - 30 to 50 of these fuel assemblies -

  targets being dispersed and loaded heterogeneously

  in the core of the fast neutron reactor.

  The casing has the same shape and dimensions as an ordinary hexagonal fuel assembly so that the target assembly can be manipulated using an ordinary machine

  handling of fuel assemblies. Bars-

  targets inside the shell contain as little as 20-50% of nuclides into minor actinides, so that the melting point and thermal conductivity of the fuel decrease due to the physical properties of nuclides to minor actinides . According to the present invention, the diameter of the target bars is less than that of an ordinary active bar, hence the increase in the heat extraction capacity, which makes it possible to avoid any reduction in the linear power. allowed. It also increases the number of target bars loaded, so you can get the

same form of assembly.

  According to the present invention, it is possible to achieve a sufficiently good annihilation of the nuclides into minor actinides, as is the case of a heart to which nuclides into minor actinides are

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  homogeneously added. In addition, it is possible to reduce the number of target assemblies to around 8-14% of the number of core fuel assemblies, resulting in a large reduction in costs relating to the production and management of fuel. The present invention will be well understood

  during the following description, made in conjunction with

  the accompanying drawings in which: FIG. 1 is a curve of the relationship between the content of minor actinides in the nuclides and the melting point of the fuel; FIG. 2 is a curve of the relationship between the content of minor actinides in the nuclides and the linear power allowed, Figure 3 is a sectional view of a fuel assembly used in the present invention, Figure 4 is a schematic view of the target fuel assemblies located in the reactor core according to the present invention, and Figure 5 is a curve of the number of active rods in a fuel-target assembly and

  the maximum linear power allowed.

  As indicated above, the present invention relates to a heterogeneously charged type of a fast neutron reactor core in which a reduced number (for example 30 to 50) of fuel-target assemblies containing nuclides to actinides

  miners are dispersed and loaded heterogeneously.

  The active target bars found in each fuel assembly contain 20 to 50% of nuclides in minor actinides and have a diameter smaller than that of ordinary active bars, so that

  can load a higher number of active bars-

  targets in the fuel-target assembly.

  In FIG. 1, the relationship between the content of minor actinides in the nuclides and the melting point of the fuel is shown. The content of nuclides

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  with minor actinides increasing, the melting point of the fuel decreases, and the thermal conductivity of the fuel also decreases. For these reasons, target fuel assemblies containing large amounts of nuclides to minor actinides are inferior to ordinary fuel assemblies (i.e., those containing no nuclides to minor actinides) in terms of the allowable linear power of the fuel, i.e. the highest linear power at which the fuel does not

not melt.

  In FIG. 2, the relationship between the content of minor actinides in the nuclides and the allowed linear power is shown. For example, when this content is 50%, the permitted linear power is about 20% lower than that obtained in the absence of nuclides into minor actinides. To avoid a reduction in the permitted linear power, the operating power must be reduced. It is therefore expected that there will not be sufficient annihilation of the nuclides to minor actinides in the

fuel-target assemblies.

  According to the present invention, this reduction in the linear power allowed can be

  avoided by reducing the diameter of the active bars-

  targets so that a greater number of active bars can be loaded into a cylinder-envelope or tube having in section the same hexagonal shape as that used with an ordinary fuel assembly for

  heart. This characterizes the present invention.

  In connection with Figure 3, there is shown an embodiment of the fuel assembly used in the present invention. Figure 3A is a cross section of an ordinary fuel assembly 10 of the core, while Figure 3B is a

  cross section of a fuel assembly-

  target 12 containing nuclides to minor actinides.

  The ordinary fuel assembly 10 has a construction

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  such that 271 active bars 16 are regularly loaded in a hexagonal cylinder or tube 14, while the fuel-target assembly 12 has a structure such that as many as 397 active bars 18 are regularly loaded in

  a hexagonal envelope tube 14 of the same geometry.

  In connection now with FIG. 4, there is shown schematically an embodiment of the arrangement of a core in which the core and the fuel-target assemblies are loaded. To obtain a power level, this arrangement is of a two-zone type in which plutonium (Pu) is more enriched in the external zone than in the internal zone. In the interior and exterior zones, there are 154 and 180 fuel assemblies, respectively, with 39 target-fuel assemblies dispersed in these zones. On the outermost side are provided fuel cover assemblies. At suitable locations are bars of

main and reserve control.

  If we take as a parameter the number of active-target bars (containing 50% of nuclides in minor actinides) in the fuel-target assembly in such an arrangement of the core, the change in maximum linear power is as shown in figure 5. The maximum linear power decreases with the increase in the number of active-target bars, and becomes lower than the linear power allowed when the number of active-target bars loaded in the fuel-target assembly reaches 397. In addition, the cavity coefficients of sodium and Doppler are equivalent to those of a homogeneous core in which nuclides of minor actinides are used in the same content; no problems are raised during operation and ordering. In addition, the annihilation rate of nuclides to minor actinides in the core of the present invention is equivalent to that

  a homogeneous heart (around 11% per year).

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  It is here that it should be noted that, when active bars are regularly inserted in a hexagonal tube-envelope, it is inevitable that their number take the finite values of, for example, 127, 217, 271, 331, 397 .... The current standard number of ordinary active bars, loaded by ordinary fuel assembly, is established at 271 taking into account the dimensions of the tube-envelope and the production of

  fuel pellets. On the other hand, all the tubes-

  envelopes must have the same shape, as can be seen in FIG. 4. According to the present invention, in which active target bars of reduced diameter are used, it is therefore possible to load a number as high as 331 or 397 active bars - targets in the same tube-envelope. However, it will be noted that it is practically impossible to use more bars, since it is not only necessary to reduce their diameter still further, but problems appear which are linked

mechanical resistance.

  In the present invention, the fuel base material containing nuclides to minor actinides is generally a mixed oxide of uranium and plutonium. However, it will be noted that the present invention can be applied to a mixed nitride of uranium and plutonium which is superior to the fuel in mixed oxides in terms of the rate of

  annihilation of nuclides into minor actinides.

  While the above embodiment has been described as using a fuel containing

  uranium and plutonium for fuel assembly-

  target, it should be noted that any fuel free from uranium and plutonium can be used to constitute the target assembly. In other words, it is possible to use for the target assembly aluminum oxide, magnesium oxide, aluminum and magnesium oxide, cerium, gadolinium oxide, aluminum nitride, or zirconium nitride, to which nuclides are added

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  into minor actinides. This excludes the possibility of nuclide formation into minor actinides from uranium and plutonium, and therefore from annihilating the nuclides into minor actinides with high efficiency. The present invention is not limited to the embodiments which have just been described, it is on the contrary subject to modifications and

  variants which will appear to those skilled in the art.

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Claims (3)

  1 - Fast neutron reactor core of the heterogeneously charged type, in which a reduced number of fuel-target assemblies (12) containing nuclides of minor actinides are dispersed and heterogeneously charged, characterized in that: shell-tube (14) for each of the target fuel assemblies has the same shape and the same dimensions as a hexagonal shell-tube for an ordinary fuel core assembly, - each of the active target bars (18) located in the tube-envelope contains 20 to 50% of nuclides into minor actinides, and has a diameter smaller than that of an ordinary active rod, and - the number of ordinary active rods loaded into an ordinary fuel assembly of   heart is 271, while the number of active bars-   targets loaded in the fuel-target assembly is 331 or 397,   - 30 to 50 of these fuel assemblies -   targets being dispersed and loaded heterogeneously in the reactor core.   2 - Reactor core as claimed   cation 1, wherein the fuel base material containing nuclides to minor actinides is an oxide   mixed or uranium and plutonium nitride.   3 - Fast neutron reactor core of the heterogeneously charged type, in which a reduced number of target fuel assemblies (12) containing aluminum oxide, magnesium oxide, d oxide aluminum and magnesium, cerium oxide, gadolinium oxide, aluminum nitride or zirconium nitride, to which nuclides of minor actinides are added, are dispersed and heterogeneously charged, characterized in what: - a casing tube (14) for each of the target fuel assemblies (12) has the same shape and the same dimensions as a hexagonal casing tube for an ordinary fuel assembly, - each of the active target bars ( 16) being in the tube-envelope contains 20 to 50% of nuclides in minor actinides, and has a diameter lower than that of an ordinary active bar, and - the number of ordinary active bars loaded in the ordinary fuel assembly is 271, while the n shadow of the active target bars loaded in the target fuel assembly is 331 or 397,   - 30 to 50 of these fuel assemblies -   targets being dispersed and loaded heterogeneously in the reactor core.